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Originally posted by kmesser
If the instance crashes, rollback is rebuilt using redo. It is not just reused from the 'pre-crash' state.
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This is correct according Oracle Documentation:

"Rollback entries change data blocks in the rollback segment, and Oracle records all changes to data blocks, including rollback entries, in the redo log. "

"If a system crash occurs, Oracle automatically restores the rollback segment information, including the rollback entries for active transactions, as part of instance or media recovery."

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Detail:

Oracle8i Concepts
Release 2 (8.1.6)
Part Number A76965-01

4 Data Blocks, Extents, and Segments

Logging Rollback Entries
Rollback entries change data blocks in the rollback segment, and Oracle records all changes to data blocks, including rollback entries, in the redo log. This second recording of the rollback information is very important for active transactions (not yet committed or rolled back) at the time of a system crash. If a system crash occurs, Oracle automatically restores the rollback segment information, including the rollback entries for active transactions, as part of instance or media recovery. Once the recovery is complete, Oracle performs the actual rollbacks of transactions that had been neither committed nor rolled back at the time of the system crash.